Paper handling devices are primarily used by large enterprises, banks, insurance companies, service-rendering enterprises, etc. In these enterprises, the paper handling devices serve to process large amounts of paper, such as in voices, reminders, statements of account, insurance policies, or cheques. For obtaining at the end of the paper handling device an appropriate combination of various necessary papers, it is necessary that, after having been printed, the various papers are correctly processed by the paper handling device. This processing takes place at successive stations of the paper handling device and comprises e.g. separating, sorting, collecting, folding and stapling of the various papers as well as subsequent enveloping of the assorted material, and stamping of the finished letter so that letters ready for dispatch are discharged at the output of the paper handling device.
The present invention refers to methods and apparatuses for folding stacks of sheets, which are adapted to be used e.g. in paper handling devices of the kind described hereinbefore. In such paper handling devices, stacks of sheets, which may comprise different numbers of sheets, are combined for the respective letters. The expression stack of sheets as used in the present context is intended to comprise an individual sheet as well as a plurality of sheets.
Depending on the fibre orientation, the kind of fibres, the basis weight, the thickness etc., a sheet of paper has a varying ease of folding, i.e. a varying folding resistance. The folding resistance of a stack of sheets is therefore defined by the ease of folding of the individual sheets forming the stack of sheets and by the number of sheets forming said stack of sheets. This folding resistance of a stack of sheets means that the folding speed used can only be a speed that is adapted to one of the above-mentioned characteristic values, viz. fibre orientation, kind of fibres, basis weight, thickness as well as the number of sheets folded at a time.
If pieces of paper which differ with regard to their ease of folding, i.e. with regard to their folding resistance, are to be processed in known devices, the folding speed must be chosen such that also the material having the least ease of folding, i.e. the highest folding resistance, can still be processed. This means that the device must always be operated at the lowest necessary speed; this will, of course, result in performance losses within the respective cycle. Modern folding mechanisms are therefore often equipped with a mechanical or an electrical adjustment mechanism with the aid of which the speed is adjusted by the operator of a device prior to a job. The expression job refers here to a production cycle in which a plurality of stacks of sheets, which may differ with regard to their ease of folding, are folded.
If, however, varying numbers of sheets are collected prior to the folding operation in a job in which paper with identical characteristic values is processed, it will be necessary to adjust also in the case of the above-mentioned operator adjustment for a job the lowest necessary speed for the whole job, and this will again result in losses within the respective cycle.